The present invention relates to a radio circuit having particular, but not exclusive, application in transportable radio communications equipment. There is a need for stable oscillator circuits to operate over a temperature range of -30 degrees C. to +70 degrees C.
A vibrating crystal has a resonating frequency at which the crystal exhibits minimum impedance. This resonating frequency varies with crystal temperature. A "stable" crystal is one in which the crystals resonating frequency is relatively constant throughout the temperature range of operation. The amount of temperature caused variance is also dependent on crystal age and environmental factors such as vibration. The higher the quality of the crystal the more stable the frequency response over a range of temperatures. In order to achieve proper frequency response, the crystal itself must be of superior quality and the temperature of the crystal must be maintained at an acceptable level.
FIG. 2 shows the drift, .DELTA.f/f, in the frequency of the crystal, expressed in parts per million, with temperature, T. in degrees Celsius. In order to be able to halt such drift it is necessary to keep the temperature of the crystal constant. The equation representing this is:
.DELTA.f/f=k.sub.a (T-T.sub.o)+k.sub.b (T-T.sub.o).sup.3
where f is the frequency of oscillation, T is the temperature of the crystal and k.sub.a, k.sub.b and T.sub.o are constants which must be individually determined for each crystal and depend, in particular, on the cut of the crystal. The frequency shift is also affected by crystal aging combined with temperature changes. The following holds true for the frequency response of aging: EQU .DELTA.f/f=k.sub.c e.sup.-k d.sup./T 1nt
where k.sub.c and k.sub.d are constants which must be individually determined for each oscillator. Thus the age of the crystal will affect the crystal's temperature response. In order to ensure homogenous response, crystals of the same type must be cured prior to use. Even though the crystal is cured the resonant frequency at a given temperature will change to a greater or lesser degree making precise compensation difficult as the required compensation temperature will vary with the age of the crystal.
The prior art contains many devices to compensate for temperature variations in radio crystals. In some cases variable pressure was applied to an axis of the crystal to stabilize the oscillating frequency during variations in temperature. Other prior art incorporates a heater inside the crystal housing, or crystal can, such as that disclosed in U.S. Pat. No. 3,818,254. One disadvantage in using a heater is the large power required to heat the crystal. Also, heater units add to the cost of the transmitter. U.S. Pat. No. 4,949,055 discloses a crystal temperature compensation circuit requiring a temperature sensor, an analog to digital converter and a microprocessor. U.S. Pat. No. 5,471,173 discloses a cascaded amplifier using current proportional to the absolute temperature. As is shown by the prior art, attempts to compensate for temperature drifting requires additional power consumption and additional circuitry, adding to the manufacturing cost of the transmitter. The present invention eliminates the need to cure or heat crystals to prevent unacceptable frequency drift due to temperature changes in the oscillating crystal.